1. Field of the Invention
The invention relates generally to the field of digital radio signal communications. More particularly, the invention relates to transmitting different bursts of a broadcast channel with different amounts of reuse and to the repeating superframe structure used to transmit the broadcast burst.
2. Description of the Related Art
Mobile radio communications systems such as cellular voice radio systems typically have several base stations in different locations available for use by mobile remote terminals, such as cellular telephones or wireless web devices. Each base station typically is assigned a set of frequencies or channels to use for communications with the remote terminals. The channels are different from those of neighboring base stations in order to avoid interference between neighboring base stations. As a result, the remote terminals can easily distinguish the transmissions received from one base station from the signals received from another. In addition, each base station can act independently in allocating and using the channel resources assigned to it.
Such radio communications systems typically include a broadcast channel (BCH). The BCH is broadcast to all remote terminals whether they are registered on the network or not and informs the remote terminals about the network. In order to access the network, a remote terminal normally tunes to and listens to the BCH before accessing the network. A remote terminal will typically scan a range of likely frequencies when it wants to access the network until it finds the strongest or clearest BCH. It will then use the BCH signal for synchronization and use information in the BCH to request access to the network.
Because the BCH is transmitted to all potential remote terminals within the range of a particular base station, it is typically broadcast omni-directionally or across a wide simultaneous directional range. This causes a great amount of interference and noise. It also consumes resources that might otherwise be used to carry traffic. Existing wireless systems employ a fixed reuse on the broadcast channel. The more resources that are used for traffic, the fewer resources there are left for traffic.
For example, a GSM system typically reuses the broadcast carrier every nine cells. An advantage of the sparse reuse pattern in GSM is the improved reliability of the broadcast channel. Because neighboring cells do not use the channel, the coverage area of the broadcast channel is much improved. This makes handover and network access more reliable. On the other hand, to find the broadcast channel in a GSM system, the user terminal must scan over at least nine frequencies, delaying handover and network access. Furthermore, the sparse reuse pattern in GSM requires that at least nine frequency resources be allocated to the broadcast channel, wasting valuable frequency resources. Recognizing the need to conserve resources, the traffic channels in GSM have a typical reuse pattern of three.
Conversely, IS-95 CDMA reuses the broadcast frequency in every cell. An advantage of this tight reuse pattern is that a user terminal can listen to just one carrier frequency and quickly determine a list of all base stations in communication range. However, in IS-95, as a cell gets loaded, the broadcast channel becomes unreliable and it is much harder for new terminals to even find the broadcast channel from the loaded cell.